1. Field of the Invention
The present invention relates to a diversity system for the mobile reception of HF-signals in different frequency ranges separated by gaps, and in particular for the mobile television reception in moving motor vehicles.
2. The Prior Art
Multi-antenna arrangements with diversity circuits are known in the prior art for receiving telecasts in moving motor vehicles, which are as interference-free as possible. The separate optimizing of the video and audio signals via a separate diversity assembly of components for each of the signals is also well known. U.S. Pat. No. 5,313,660 is cited in this connection as an example of the state of the art in this field: Several antennas, such as, for example, four antennas are connected to two antenna change-over switches via a hybrid circuit, wherein the one circuit is controlled by a diversity circuit for the video signals and the other by a diversity circuit for the audio signals. In this way, the best-possible quality of the amplitude-modulated video signals and the frequency-modulated audio signals is simultaneously achieved with the lowest expense with respect to the number of antennas used.
In view of the quality, this diversity principle is adequately supplemented by the invention according to German Patent Application P 3,926,336, by which the video signals that are received can be detected in the line circle within the horizontally synchronized pulse. Reversing between antennas is possible in the same cycle, and the most favorable signal can always be switched through to the receiver.
However, there are problems that occur when trying to achieve an enhancement and stabilization of video reception by a quick antenna changeover. The antenna to which the signal path is switched to, supplies in accordance with the selection circuits, a higher signal level than the one that was received and previously viewed. The difference between the level values is usually accompanied by a different phase position. The automatic gain control (AGC) of the receiver however, is not capable of quickly adjusting itself to the changed level without some delay. Moreover, the filters and demodulator resonant circuits also require a certain finite time to adjust to the new phase position. This may cause interference with the first line after the new antenna has been switched through. The following lines also often show deviations in brightness and contrast until the AGC circuit adapts to the new level value. Thus, the reproduction quality is impaired by the diversity function.